1. Field of the Invention
This invention relates to coated natural leather and, more particularly, to leather coated on the flesh side with polyurethanes or polyurethaneureas.
2. Description of Background Art
Coatings are generally applied to natural leathers on the grain side of the leather for aesthetic, waterproofing, or durability purposes. Although some applications of polymers to the flesh side of leather have been disclosed, the results have not been satisfactory with regard to stretch and recovery.
French Patent Number 2,129,056 discloses a method for reinforcing leather by coating the flesh side with a rigid, crosslinked polyurethane elastomer, the polymer being formed in situ on the leather. Shoes made from the coated leather are also disclosed but, because the polymer is rigid, neither the leather nor the shoes made therefrom have the desirable stretch and recovery needed for comfortable wear.
German Patent Number 1,469,530 discloses a process for impregnating an unspecified side of natural leather with a solution of an irreversibly heat-hardenable resin (e.g. polyurethane), followed by deep-drawing or vacuum-shaping and heating to a permanent form having shape stability. Leather processed in this way will not have a desirable combination of stretch and low set, because it will be hardened into a permanent form.
French Patent Number 866,856 discloses coating the flesh side of dyed leather with dilute solutions of chlorinated rubber to bind leather dust to the leather. The properties of the leather, however, are not affected by this treatment.
Romanian Patent Number 104,707 discloses the use of crosslinkable polyurethane prepolymers to coat a variety of materials, but specifics of their application to leather are not disclosed.
U.S. Pat. No. 3,027,276 discloses lightly spraying a thin, low-solids solution of a diisocyanate prepolymer onto the flesh side of dyed or pigmented suede containing free moisture to prevent crocking. In this method, curing is completed on the leather by reaction with environmentally available water. This method does not alter the mechanical properties of the treated suede.
U.S. Pat. No. 5,932,056 discloses a method of laminating a one-way stretch fabric in alignment with natural leather, but such laminates can be too thick for some uses.
French Patent Number 1,589,164 discloses reinforcing the back of thin or split skins by coating the flesh side of leather with an aqueous emulsion of thermoplastic acrylic and vinyl polymers without significant penetration of the leather. The abrasion and water resistance of the leather is improved. However, such a coating will not provide sufficient recovery to avoid permanent distortion after the coated leather is highly stretched.
A combination of high stretch (for comfort and ease of shaping), low set (for shape retention), high air permeability, and visual aesthetics of natural leather is still needed.
The coated natural leather of the present invention comprises
(a) natural leather having at least about 15% elongation prior to coating; and
(b) 5-70 wt %, based on the weight of uncoated leather, of elastomeric polyurethane, from a polyurethane solution having a viscosity of 1,000-300,000 centipoise, coated on the flesh side of the leather which is capable of absorbing the polyurethane solution, wherein the coated leather has an elongation of at least about 15%, a set of no more than about 25%, and a measure of deformation (ratio of fifth cycle set to fifth cycle elongation) at least 0.05 lower than the uncoated leather.
It has now been discovered that natural leather which has been coated on the flesh side with a solution of an elastomeric polyurethane has low percent set relative to its elongation. This unexpected combination of properties means that the coated leather retains its softness and stretch (for comfort) but resists permanent deformation for good fit retention. It has also been discovered that the solution-coated leather retains surprisingly high elongation and low set after extended storage. Further, the polyurethane solution-coated leather of the invention displays highly desirable soft-leather tactile aesthetics on the (coated) flesh side. The coated leather of the invention can be made into articles such as footwear, apparel such as jackets and pants and apparel accessories such as purses, belts and gloves, upholstery, and luggage.
As used herein, xe2x80x9cnatural leatherxe2x80x9d means the tanned or partially tanned skin of any suitable animal. xe2x80x9cFlesh sidexe2x80x9d means the inside of the leather or skin when it is still part of the animal, and xe2x80x9cgrain sidexe2x80x9d means the outside of the leather or skin when it is still part of the animal. When the leather is a split-grain leather, xe2x80x9csplit sidexe2x80x9d means the side that was within the leather or skin before it was split. By xe2x80x9ccoated on the flesh sidexe2x80x9d is meant coating the flesh side of full grain (un-split) leather or the split side of split grain leather. xe2x80x9cSetxe2x80x9d means the percent residual extension of a measured length of coated or uncoated leather after it has been stretched at a force of 17.5 Newtons per centimeter (N/cm, of sample width) at room temperature and allowed to relax to substantially zero force.
Elastomeric polyurethanes useful in this invention can be prepared by reacting a polymeric glycol with a diisocyanate to form a capped glycol, dissolving the capped glycol (in a suitable solvent), and then reacting the capped glycol with a difunctional chain extender having active hydrogen atoms. Such polyurethanes are termed xe2x80x9csegmentedxe2x80x9d because they are comprised of xe2x80x9chardxe2x80x9d urethane and urea segments derived from the diisocyanate and chain extender and xe2x80x9csoftxe2x80x9d segments derived primarily from the polymeric glycol. Suitable solvents for preparing solutions of such polymers are amide solvents such as dimethylacetamide (xe2x80x9cDMAcxe2x80x9d), dimethylformamide (xe2x80x9cDMFxe2x80x9d), and N-methylpyrrolidone, but other solvents such as dimethylsulfoxide and tetramethylurea can also be used.
Polymeric glycols used in the preparation of the elastomeric polyurethanes include polyether glycols, polyester glycols, polycarbonate glycols and copolymers thereof. Examples of such glycols include poly(ethyleneether) glycol, poly(trimethyleneether) glycol, poly(tetramethyleneether) glycol, poly(tetramethylene-co-2-methyl-tetramethyleneether) glycol, poly(ethylene-co-butylene adipate) glycol, poly(2,2-dimethyl-1,3-propylene dodecanedioate) glycol, poly(3-methyl-1,5-pentamethylene dodecanedioate) glycol, poly(pentane-1,5-carbonate) glycol, and poly(hexane-1,6-carbonate) glycol.
Useful diisocyanates include 1-isocyanato-4-[(4xe2x80x2-isocyanatophenyl)methyl]benzene, 1-isocyanato-2-[(4xe2x80x2-isocyanato-phenyl)methyl]benzene, isophorone diisocyanate, 1,6-hexanediisocyanate, and 2,4-tolylene diisocyanate, and mixtures thereof.
The chain extender can be a diol or a diamine. Useful diols include ethylene glycol, 1,3-trimethylene glycol, 1,4-butanediol, and mixtures thereof. Diol chain extenders lead to polyurethanes. Useful diamines include ethylene diamine, 1,2-propanediamine, 2-methyl-1,5-pentanediamine, 1,3-diaminopentane, 1,4-cyclohexane-diamine, 1,3-cyclohexanediamine, and mixtures thereof. In this case, the polymer produced is a polyurethaneurea. When a polyether glycol and a diamine chain extender are utilized, the polymer produced is a polyetherurethaneurea; when a polyester glycol is utilized in combination with a diamine chain extender, a polyesterurethaneurea is produced. Monofunctional amine chain terminators such as diethyl amine, butylamine, cyclohexylamine, and the like can be added to control the molecular weight of the polymer.
Additives, for example, antioxidants and lubricants, can be added in small quantities to the polyurethane coating solution, provided such additives do not detract from the benefits of the invention.
Leather from sheep, goat, cattle, calf, pig, ostrich, kangaroo, elephant, deer, lizard, crocodile, snake, and the like can be used in the present invention. The characteristics of natural leather can vary greatly depending on the animal source, the part of the animal it has been taken from, and the method of tanning. In the event the natural leather does not have adequate stretch (at least about 15% elongation, at 17.5 Newtons per centimeter applied force), the desirably high elongation of the coated leather of the invention cannot be obtained.
In the present invention, the elastomeric polyurethane coating comprises about 5-70% polyurethane by weight of uncoated leather; preferably about 15-55 wt %. At 17.5 Newtons per centimeter applied force, the coated leather has an elongation of approximately at least about 15% and preferably at least about 20% and a set of approximately no more than about 25%, preferably no more than about 20%. The coated leather of this invention must have a measure of deformation (ratio of fifth cycle set to fifth cycle elongation) of at least approximately 0.05 better (lower) than the uncoated (control) leather and, preferably, a measure of deformation value approximately no more than about 0.5.
The viscosity of the polyurethane solution can influence the degree of penetration of the polymer into the leather and the amount of polymer deposited. When viscosity is too low, insufficient amounts of elastomer can be deposited in the leather and excessive penetration to the grain side of the leather can occur. When solution viscosity is too high, penetration of the solution into the leather can be reduced, thereby inhibiting bonding of the elastomer to the leather. This can reduce the improvement in the set of the leather of this invention provided by the elastomer. The solution of elastomeric polyurethane to be coated onto the natural leather has a solution viscosity of approximately 1,000-300,000 centipoise (xe2x80x9ccPsxe2x80x9d), preferably 5,000-200,000 cPs, as measured at about 22xc2x0 C.
When natural leather (including full-grain, suede, nubuck and split-grain leather) is coated with polyurethane only on the flesh side, the grain (face) side will retain its natural grain (or, in case of suede and nubuck, its brushed) look and feel. (This face side can be sanded and/or brushed to achieve nubuck or suede appearance.) However, leather can also be coated with a polyurethane solution on both the flesh and grain sides if modification of the appearance and feel of the grain (face) side is desired, provided the desirable elongation and set characteristics of the coated leather are not deleteriously affected.
It is necessary for the leather to be coated in the present invention to be able to absorb a polyurethane solution. Therefore, no treatment, such as coating or drying, can be carried out that can restrict the penetration of the polyurethane into the leather excessively. The leather can be chrome-tanned or vegetable tanned, and it can be dyed.
The natural leather to be utilized in this invention can be of any thickness. Thicker leather can require more elastomer to be deposited and longer coating times to allow the polymer solution to penetrate the leather than thin leather, which is preferred for some uses due to its softness and stretchability. As an example of a suitable thin leather, the bovine leather used in ladies"" dress shoe uppers is generally in the range of about 0.8-1.0 mm thickness.
The process of the present invention for manufacturing coated leather comprises the steps of providing natural leather having at least about 15% elongation and being capable of absorbing polyurethane solution; coating an elastomeric polyurethane solution, viscosity of about 1,000-300,000 cPs, preferably 5,000-200,000 cPs (measured at about 22xc2x0 C.) onto the flesh side of the leather; and drying the coating such as by evaporating the solvent, preferably at no higher than about 75xc2x0 C., wherein the coating on the coated leather comprises approximately 5-70 wt %, based on the weight of the uncoated leather.
The polyurethane solution can be applied at the xe2x80x9cwet bluexe2x80x9d stage of tanning, that is after chrome tanning and before retanning, coloring, or fatliquoring, drying, etc. Some polyurethane, however, can be lost during subsequent processing, with consequent increase in the set. Application of polyurethane at the early stages of the tanning operation is preferably done after the leather has been split to the desired thickness. Prewetting the side of the leather to be coated with the solvent utilized in the preparation of the polyurethane solution can help adhere the polymer to the leather.
Any suitable method of coating the polyurethane solution onto the leather can be used. In general, the thickness of the coating can be controlled by utilizing a coating implement held at a predetermined distance above the leather. The solution can also be mechanically pressed into the flesh side of the leather, thus improving the bond between the elastomeric polyurethane and the leather and, also, between different portions of the subsequently dried coating. Rollers, platens, scrapers, knives, and the like can be used in the process of this invention as coating implements, as well as coating machines such as those ordinarily used in processing leather. Spraying the solution onto the flesh side of the leather can be effective, especially if the force of the spray is sufficient to result in good penetration and bonding.
It can be advantageous deliberately to leave portions of the leather surface uncoated to improve the porosity of the coated leather. For example, a periodically or randomly interrupted coating can be desirable to generate a variety of patterns, such as a net pattern. The porosity of the coated leather can also be improved by lightly brushing the coated side, for example with a wire brush.
Depending on the natural leather being used, shrinkage of the leather can occur during the removal of solvent from the coated leather, with consequent deterioration of the aesthetics of the coated leather. In such circumstance, solvent can be evaporated while the coated leather is held under low but sufficient tension to prevent such shrinkage. If too much tension is applied, the leather can become excessively stretched, which can result in loss of elongation in the coated leather. Drying the coated leather at too high a temperature can adversely affect the leather by degrading it, and the polyurethane solution can be driven through to the grain side. It is preferred that drying temperatures no higher than about 75xc2x0 C. be used in the process of the invention.
In Examples 1-4, the polymer source for the polyurethane solution was chopped LYCPA(copyright) Type 136C spandex (a polyetherurethaneurea-based spandex; a registered trademark of E.I. du Pont de Nemours and Company). Unless otherwise noted, a Rotolab 400 coater was used to coat the solution onto the flesh side of the leather (Gemata S.P.A., Trissino, Italy). The coating roll had a recessed patterned surface, increasing the amount of polyurethane coated onto the leather.
In all Examples, the polymer solution was coated onto the flesh side of the leather, leaving the grain side of the leather uncoated; coating wt % is based on the weight of uncoated leather.
Unless otherwise noted, the elongation and set of uncoated and coated leather were measured using an Instron instrument (Instron Ltd., High Wicomb, UK) and Instron Series XIII software was used during five stretch-and-relax cycles. The measurements were made on the long direction of 2 cmxc3x9710 cm samples. The 10-cm length corresponded to the high-stretch direction of the leather. Elongation (%) values were recorded at an applied force (stress) per unit sample width of 17.5 Newtons per centimeter (N/cm) on the stretch part of the fifth stretch-and-relax cycle. Elongation of uncoated leather was measured on samples adjacent to that portion of the leather from which samples were cut out for coating. Set (%, residual elongation) was measured by stretching the leather at room temperature to 17.5 N/cm applied force and allowing it to relax. The final, relaxed length of the leather sample was measured immediately after relaxation to zero applied force. Set was calculated as follows:
Set (%)=relaxed lengthxe2x88x92initial lengthxc3x97100/initial length
Relaxed length was measured immediately after the fifth stretch-and-relax cycle.
To be a coated leather of this invention, the measure of its deformation must be lower than that of the uncoated leather (control) by at least about 0.05 unit. The greater the potential elongation in the leather, the more susceptible it can be to high set. This ratio of set to elongation is reported in the Tables as a dimensionless number.
In the Examples, the permeability to air of the coated leather was measured with a TEXTEST Fx 3300 calibrated anemometer (Textest AG, Zurich, Switzerland) using an air pressure of 600 Pa on a 20-cm2 area of the leather and was reported in liters/m2/second and solution viscosities were measured at 22xc2x0 C. (unless otherwise stated) with a Brookfield Digital Viscometer, Model DV-II.